(a) Field of the Invention
The present invention relates to bridges and particularly to suspension bridges. It further relates to cable carrying structures and a method of bridge construction.
(b) Description of Prior Art
Known suspension bridges normally have towers set in water a distance from shore, the present invention replaces such towers by inclined spars or booms leaning from shore and over the water to points corresponding to tower tops. The normal arrangement of cables for a suspension bridge is replaced in the present invention by cable systems. In one embodiment according to the invention, a first system helps to support the lean of the spars. A second system helps to support a length of deck centered under the spar tops, half the deck length extending to the spar base at shore, the other half, a counter-weight extending oppositely an equal distance beyond the spar top. This second system accordingly supports a deck portion near shore, somewhat like an elongated swing, suspended by several inclined hangers from the spar tops. A third system supports all the deck at mid span lying between the extremities of the second system at both shores. Oscillation of the deck is dampened by overlapping the suspension systems. This embodiment and others are discussed hereinafter in detail together with other aspects of the invention.
Suspension bridges have been known from antiquity as made of twisted hemp or vines for ropes, where a single rope served for the feet and two higher ropes for the hands. Sometimes a second foot rope permitted having cross-pieces tied, for a deck, and such primitive bridges still are made.
Modern suspension bridges normally have two towers set in water, spaced from shore about a sixth to a fourth of the water width, both towers supporting at their tops two or more cables draped from shore-to-shore. Hung off these cables are secondary cables supporting a traffic deck. Metal is put to such effective use in suspension bridges, compared to other bridge types, that the light weight is prone to oscillation. A common practice therefore is to stiffen against oscillation by utilizing inclined cable bracing or trusses or both. The stiffening does not contribute directly to spanning a gap but the added weight of stiffening is a necessary burden for practical bridging. Stiffening is retained in the present invention, but the stiffening is used to contribute directly toward spanning a gap.
Known suspension bridges have another burden or problem, namely piers in a watercourse, that the present invention is directed toward alleviating. Piers somewhat obstruct river and tidal flow, especially ice flow, all aggravated by temporary structures needed during bridge construction. Also, piers and their foundations must be positioned and constructed with reserve capacity to withstand ice forces, flow turbulence and especially an undercutting or scouring tendency. The reserve capacity increases the size and cost of foundations and piers.
Piers also are a navigation hazard posing risk of losing a ship or a bridge itself and lives of travellers. Ship collisions with piers and bridges have resulted from poor visibility, storms, treacherous currents, unclear signals and human error. A bridge collapse not only blocks crossing and shipping traffic, but brings hardship to a region, and poses strategic problems for military and naval authorities. Piers in a busy seaport or lake therefore are a concern for port authorities, and periodically are subjects of public debate, with stress on security.
A further problem of known suspension bridges is that cables have a uniform cross section, thus uniform strength, but cable tension changes along its length from a maximum at each tower to a minimum at mid span. As is well known, tapering of cables if achievable would proportion the weight of cable to the load at progressive points along the cable, thus bringing economy in cable weight and cost. The present invention addresses this problem, not directly by tapering the cables, but by distributing load to different systems of cables as will be described. As is well known, the ultimate strength of a given weight of wire increases as it is drawn thinner, and an economical size for conventional suspension-bridge cables is found to be about 0.20 inch (5 mm) diameter because the thicker the wire the less length there is to be spun to make up a cable of the necesary diameter. In the present invention, the subdivision of loads, and the separate support of these loads by individual systems of cables all act as a substitute for tapered cables. Therefore, less wire is needed than for conventional main cables, and the use of thinner wire becomes practical with greater strength for a given weight of wire.